This invention concerns sewage treatment processes, and in particular relates to scum removal in a clarifier.
In a typical conventional activated sludge sewage treatment process, a considerable volume of scum and other gross floatables enters with the influent feed into the clarifier basin. This is distinct from the biological scum which arises from biological processes occurring in the basin. In conventional clarifiers the influent scum and other floatables were often moved out of the influent well to be collected along with biological scum, over the entire clarifier surface or at the periphery of the clarifier. The two types of scum typically were commingled and discharged together, and where influent well scum and floatables were collected they were delivered to the periphery of the clarifier and commingled with biological scum.
Examples are the SS Clarifiers of Enviroquip, Inc., being of open trough design and having scum ports that move with the rake arms collecting scum and other floatables over the entire clarifier surface, with commingling. Although such systems function well to remove scum from the clarifier, a problem is the volume of water taken in with the scumxe2x80x94sometimes 200 to 300 gallons per minute, producing a very watery, diluted scum which would hydraulically overload the digester or sludge concentration area in the wastewater handling area.
Some prior clarifier systems have had influent wells that prevented egress of scum or gross floatables at the surface. Some have included a fixed scum beach or box within the influent well, with a rotating scraper blade in the well for collecting surface scum and floatables onto the beach or box. Some clarifiers have incorporated rotating open scum troughs extending from center pier to the periphery of the clarifier with ports both inside and outside the influent well. However, even where the scum and floatables were sequestered and collected within the influent well, they were not handled separately from other biological scum for separate discharge and they were not discharged through the center pier.
The following U.S. patents were concerned with sewage treatment and clarifiers and have some relevance to the subject matter of this invention: U.S. Pat. Nos. 1,892,688, 2,295,982, 2,262,412, 2,506,927, 2,661,094, 2,681,151, 2,798,041, 2,801,007, 2,822,928, 2,875,697, 3,132,733, 3,166,502, 3,206,036, 3,216,570, 3,234,880, 3,314,547, 3,327,867, 3,770,131, 3,892,666, 3,926,805, 3,396,102, 3,487,017, 3,526,591, 4,193,877, and 5,219,470.
The prior art did not contemplate the efficient scum removal system of the present invention, in which virtually all scum and other floatables entering the clarifier""s influent well are sequestered in the influent well, efficiently collected there in a nearly dry state, then discharged down the center pier and separately from other scum or sludge in the clarifier, for separate subsequent processing.
In the clarifier system of the invention all access areas at the surface of the influent well are blocked off, and the well is used as a basin and scum containment area. This allows about 90% to 95% of scum to be captured where it enters the clarifier.
Principal innovation of this clarifier system is to expand the function of the influent well to include capturing virtually all inorganic gross floatables and scum inside the influent well, by providing an additional scum collection device within the influent well, and the collection device thus feeding an isolated waste stream which is piped down the center pier for optimal disposal of this waste stream separate from the biological scum collected from the clarifier surface outside the influent well.
Instead of removing scum using a series of scum ports at the leading edge of a sludge removal trough or scum trough as in some prior clarifiers, a preferred embodiment of the clarifier of the invention has within the influent well an inclined scum ramp at the leading edge of each rotating scum removal trough, behind which is the scum collection trough. The clarifier rake arms pass through an arc of rotation, preferably 360xc2x0, building up scum and other floatables as the ramps skim the surface. The scum and other floatables accumulate on the surface and at the beach formed by the ramp and are pushed forward by the approach ramp. Once per revolution each moving scum ramp approaches a stationary scum surface blade which is secured to fixed structure above the influent well but which can pivot up when engaged against the ramp.
The scum blade, extending partially under the liquid surface, provides a barrier to the forwardly pushed scum and contacts and slides up the moving ramp, sweeping the scum up the ramp and into the scum collection trough.
This influent well scum collection device is essentially opposite to the rotating blade/fixed scum beach that has been employed in some prior clarifier influent wells. Moreover, even those devices failed to provide for a separate waste stream of the well-collected scum and did not discharge down the center pier.
By reducing the volume of liquid in the scum collection system, the described arrangement greatly reduces the amount of free water with the scum as compared to typical scum port designs. The new system permits the scum in the influent well, including other floatables, to be trapped and contained with little liquid, prior to being disposed of and treated separately from biological scum captured elsewhere in the clarifier basin, and without commingling with sludge or returning to the head of plant.
Although the well scum is efficiently collected using the described scum ramp sweeping the influent well, other systems operable within the influent well, such as ports or slide gates, could alternatively be used. One alternative sometimes preferable is an open trough with scum ports which terminates at the influent well, used in the same clarifier system with a traditional scum box mounted at the periphery of the clarifier to capture biological scum to be handled separately. The open scum trough within the influent well conducts the collected scum and inorganic floatables into a waste stream which exits the center pier separately.
The system of the invention thus provides for a cleaner water surface outside the clarifier""s influent well, cleaner effluent from the clarifier, and segregation of gross floatables and scum from biological scum, thereby allowing optimal disposal or use of each waste stream. In addition, the system contains influent well scum even when the scum pumps are idle and provides for very little dilution of gross floatables and scum captured in the influent well.
It is thus among the objects of the invention to make more efficient the operation of a clarifier and other systems in an activated sludge sewage treatment process, by collecting most of the scum of the clarifier system as scum enters the influent well with the influent feed, and by discharging this scum and other floatables in a dedicated stream down through the center pier with very little volume of water. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings.